Hydraulically controlled actuator for actuating a valve

ABSTRACT

An hydraulically controlled actuator ( 16 ) for activating a valve ( 10 ) is proposed, especially for activating a gas-exchange valve ( 10 ) in a combustion cylinder ( 11 ) of an internal combustion engine, which includes two fluid-filled pressure chambers ( 21, 22 ) having controllable chamber volumes and a movable operating piston ( 18 ) which delimits the pressure chambers ( 21, 22 ) by piston sides facing away from one another, the operating piston ( 18 ) acting upon the valve ( 10 ) and having an effective closing area acted upon by fluid pressure in the pressure chambers ( 21, 22 ) to close the valve ( 10 ) and an effective opening area acted upon by the fluid pressure to open the valve ( 10 ). To influence the kinematics of the opening and closing movement of the valve ( 10 ), the operating piston ( 18 ) is designed such that the surface area of at least one of the two effective areas changes along the sliding path of the operating piston ( 18 ).

BACKGROUND INFORMATION

[0001] The present invention is directed to an hydraulically controlledactuator for activating a valve, especially a gas-exchange valve in acombustion cylinder of an internal combustion engine according to thedefinition of the species in Claim 1.

[0002] Such hydraulically controlled actuators are used in devices forthe electro-hydraulic valve control of intake and exhaust valves incombustion cylinders of internal combustion engines, one actuator ineach case being assigned to one gas exchange valve used as intake ordischarge valve.

[0003] In a known device for controlling a gas-exchange valve (DE 198 26047 A1), the operating piston connected to the valve tappet of thegas-exchange valve is guided in a working cylinder in an axially movablemanner and, by its end faces which face away from one another, delimitsthe two pressure chambers formed in the working cylinder. While the onefirst pressure chamber, via which a piston movement in the direction ofvalve closing is effected, is always acted upon by pressurized fluid,the other second pressure chamber, via which a piston movement in thedirection of valve opening is effected, is selectively acted upon bypressurized fluid or discharged again to approximately ambient pressurewith the aid of solenoid valves. The pressurized fluid is provided by acontrolled pressure supply. The solenoid valves are embodied as 2/2directional control valves, a first solenoid valve connecting the secondpressure chamber to the pressure supply, and a second solenoid valveconnecting the second pressure chamber to a discharge line. In theclosed state of the gas-exchange valve, the closed solenoid valveseparates the second pressure chamber from the pressure supply, and theopen second solenoid valve connects it to the discharge line, so thatthe operating piston is brought into its closing position by the fluidpressure prevailing in the first pressure chamber. To open thegas-exchange valve, both solenoid valves are energized. Due to theswitching solenoid valves, the second pressure chamber is blocked fromthe discharge line and connected to the pressure supply. Thegas-exchange valve opens, the magnitude of the opening lift beingdependent on the formation of the electric control signal applied to thefirst solenoid valve, and the opening speed being dependent on the fluidpressure input by the pressure supply. In order to keep the gas-exchangevalve in a particular open position, the first solenoid valve issubsequently de-energized, so that it once again separates the secondpressure chamber from the current supply. To close the gas-exchangevalve, the second solenoid valve is de-energized. As a result, thesecond pressure chamber is connected to the discharge line, and thefluid pressure prevailing in the first pressure chamber guides theoperating piston back into its valve-closure position, the valve thusbeing closed by the operating piston. In this manner, it is possible touse an electric control device to generate control signals for thesolenoid valves by which any desired valve-opening position of thegas-exchange valve are able to be adjusted.

SUMMARY OF THE INVENTION

[0004] The hydraulically controlled actuator according to the presentinvention for actuating a valve, having the features of Claim 1, has theadvantage over the related art that by a defined change in the effectiveopening and/or closing area of the working piston, the kinematics of theopening and/or closing movement of the valve are able to be controlledwithin broad limits in a very precise manner as a function of theoperating piston's sliding path. For instance, during the openingprocedure of the valve, a high adjusting force may first be generated onthe valve for a fraction of the total valve lift. This high adjustmentforce is then markedly reduced again for the remaining lift of thevalve. Such an opening characteristic curve is of great advantageespecially in the case of gas-exchange valves in combustion cylinders ofan internal combustion engine; for, in particular on the discharge sideof the combustion cylinders, there is a need for an initially highopening force of the actuator, so that the gas-exchange valve may openagainst the residual gas pressure in the combustion cylinder. If,following a pressure compensation between combustion chamber anddischarge channel, the actuating force is then lowered for the furtheropening operation of the valve, the energy required for the openingtravel of the gas-exchange valve is considerably reduced. Overall, it ispossible to reduce the energy consumption of an electro-hydraulic valvecontrol within the valve lift by optimizing the change in the effectiveopening area in accordance with the particular requirements.

[0005] Furthermore, the solenoid valve determining the opening onset ofthe gas-exchange valve and the maximum lift of the gas-exchange valvemay be designed for a smaller flow rate. The reason for this is that,upon initiation of the valve opening procedure by the closing of thesecond solenoid valve toward the discharge line and the opening of thefirst solenoid valve to the pressure supply, at first only enough fluidflows into the second pressure chamber to raise the pressure in thesecond pressure chamber. As soon as the opening force, resulting fromthe pressure and effective opening area, overcomes the existingfrictional forces, the operating piston begins to move in the openingdirection of the gas-exchange valve. In the process, the flow ratethrough the first solenoid valve resulting from the expansion of thechamber volume in the second pressure chamber does not rise abruptly,but steadily from zero to a maximum value. The large effective openingarea of the operating piston thus is effective at a time when the flowrate through the open first solenoid valve has not yet reached itsmaximum value. The reduction in the effective opening area sets in earlyenough to limit the maximum flow rate through the first solenoid valveto a low level. This level is less than the level that would result ifthe effective opening area of the operating piston were kept constantover the lift.

[0006] According to the present invention, the closing operation of thevalve also may be advantageously influenced by the formation of theeffective closing area of the operating piston as a function of itssliding path in that the valve member, in the course of the pistonmovement, sets down on the valve seat with a reduced closing force, as aresult of a timely reduction in the effective closing area of theoperating piston. This advantage is particularly important for theactivation of gas-exchange valves in combustion cylinders of an internalcombustion engine. For there is a need, especially on the intake side ofthe combustion cylinder, both for a rapid closing of the intake valveand also for a low striking speed of the valve member on the valve seaton the side of the combustion cylinder. This striking speed, for noiseand wear reasons, must not exceed certain limiting values, for instance,approximately 0.5 m/s at idling speed and approximately 0.5 m/s atmaximum speed. By the reduction of the effective closing area of theoperating piston shortly before reaching the closing position of thegas-exchange valve, as proposed by the present invention, the openingforce of the actuator is reduced, thereby making a first contributiontowards observing these limiting values.

[0007] The measures specified in the further claims permit advantageousfurther developments and improvements of the hydraulically controlledactuator indicated in Claim 1.

[0008] According to an advantageous embodiment of the present invention,the operating piston is designed in such a way that, when the operatingpiston moves out of its valve position, the effective opening area ofthe operating piston is reduced by a predefined amount following atleast one predefined sliding path.

[0009] According to a preferred embodiment of the present invention,this is realized by the operating piston having a multi-part design andbeing made up of at least two concentric partial pistons which havedifferent axial lengths and are able to be moved relative to oneanother. They are inserted into one another in such a way that thesecond pressure chamber is delimited by all front faces and the firstpressure chamber only by a part of the front faces of the partialpistons. The sliding path of the at least one partial piston notdelimiting the first pressure chamber is reduced relative to the overallsliding path of the operating piston, the reduction occurring in astepped manner in the case of more than two partial pistons.

[0010] According to an alternative embodiment of the present invention,the operating piston is designed in such a way that, when the operatingpiston moves out of its valve-closure position, the effective openingarea is larger in the leading area of the sliding path than it is in therest of the sliding path. When the operating piston is moved out of itsvalve-opening position, the effective closing area in the end area ofthe sliding path is smaller than it is in the rest of the sliding path.

[0011] According to an advantageous embodiment of the present invention,this design of the operating piston is realized in that the operatingpiston is embodied as a stepped piston having a plurality of pistonsections with different diameters. The operating piston has a centerpiston section which has the largest diameter; a lower inner pistonsection, which continues from the center piston section and extendsthrough the first pressure chamber and has a smaller diameter than thecenter piston section; an upper inner piston section which continuesfrom the center piston section and extends through the second pressurechamber and has a reduced diameter compared to the diameter of the lowerinner piston section; and an outer piston section which in each case islocated at an end of the inner piston sections and whose diameter is ineach case larger than the diameter of the adjoining inner pistonsection.

[0012] This design of the operating piston does not only realize aneffective opening and closing area of the operating piston that changesin a defined manner across the sliding path of the operating piston, butin the case of closing also achieves a secondary effect which, inaddition to the effective closing area of the operating piston which isreduced prior to the end of the closing movement, contributes to thereduction in the closing force. Due to the described graduated diameterdesign of the piston sections of the operating piston, the diameter ofthe operating piston in the second pressure chamber changes shortlybefore the end of the closing movement and the piston area delimitingthe second pressure chamber is thus increased. This causes an increasein the discharging fluid stream. The open second solenoid valve, whichat this time acts as a constant throttle toward the discharge line,opposes this increased fluid flow by an increased back-pressure, so thatthe pressure force acting in the first pressure chamber in the closingdirection, which is reduced shortly before the end of the closingmovement, is met by a rapidly generated counter force acting in anopposite direction. This counter force brakes the operating piston andin this way makes it possible to attain the aforementioned limitingvalues for the striking speed of the valve member on the valve seat as aresult of the reduced closing force of the operating piston.Consequently, special devices for reducing the striking speed of thevalve member on the valve seat of the gas-exchange valve, which havebeen used until now, may be dispensed with.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention is elucidated in the following on the basisof exemplary embodiments depicted in the drawing.

[0014] The figures show in schematic illustration:

[0015]FIG. 1 a circuit diagram of a device for controlling agas-exchange valve including an actuator, shown in longitudinal section,for actuating the gas-exchange valve, which is represented in apart-sectional longitudinal view;

[0016]FIG. 2 a longitudinal section of an actuator for actuating agas-exchange valve according to an additional exemplary embodiment.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0017] In the device for controlling a gas-exchange valve in acombustion cylinder of an internal combustion engine, shown in FIG. 1 asa circuit diagram, gas-exchange valve 10 controls an opening-crosssection 12 in a combustion cylinder 11, which is indicated in FIG. 1 bya section of its cylinder wall. Gas-exchange valve 10 may be used as anintake valve for controlling an intake cross section, and as a dischargevalve for controlling a discharge cross section in combustion cylinder11. Gas-exchange valve 10 includes a valve tappet 13 at whose one end aplate-shaped valve-sealing surface 14 is situated which, in order tocontrol opening cross section 12, cooperates with a valve-seat surface15 which is formed on the cylinder wall of combustion cylinder 11 andencloses opening-cross section 12. To open gas-exchange valve 10,valve-sealing surface 14 is lifted off from valve-seat surface 15 to agreater or lesser extent by the movement of valve tappet 13. To closegas-exchange valve 10, valve-seat surface 14 is pressed firmly ontovalve-seat surface 15 by the opposing movement of valve tappet 13.

[0018] An hydraulically controlled actuator 16 is provided to open andclose gas-exchange valve 10; it has a working cylinder 17 and anoperating piston 18 which is guided in working cylinder 17 so as to beaxially movable. In the exemplary embodiment of actuator 16 in FIG. 1,working cylinder 17 is realized by a bore introduced in a housing 19into which a guide sleeve 20 is inserted to guide operating piston 18and which is appropriately sealed at the front end. Operating piston 18,which is fixedly connected to valve tappet 13, divides working cylinder17 into two hydraulic pressure chambers 21, 22 which are bounded by itat front faces that face away from one another. Lower first pressurechamber 21 has a connection piece 211, and upper second pressure chamber22 has two connection pieces 221, 222. Via connection pieces 211, 221,222, the two pressure chambers 21, 22 are filled with a fluid, such ashydraulic oil. For this purpose, connection piece 211 of first pressurechamber 21 is connected to a controllable pressure-supply device 24 byway of a pressure line 23, and connection piece 221 of second pressurechamber 22 is connected thereto via a first solenoid valve 25, whileconnection piece 222 of second pressure chamber 22 is connected, via asecond solenoid valve 26, to a discharge line 27 which leads to a fluidreservoir 28. Both solenoid valves 25, 26 are embodied as two-waydirectional control valves having spring readjustment, which areactivated for their switching by an electronic control device (not shownhere). In the rest position, or basic position, of both solenoid valves25, 26, which is shown in FIG. 1, second pressure chamber 22 isseparated from pressure-supply device 24 and connected to discharge line27. The fluid pressure prevailing in second pressure chamber 22corresponds approximately to the ambient pressure.

[0019] Pressure-supply device 24 includes a controllable high-pressurepump 29 which draws in fluid from fluid reservoir 28; a check valve 30and a reservoir 31 to dampen pulsations and store energy. A permanenthigh pressure, which is input into first pressure chamber 21, is presentat output 241 of pressure-supply device 24, to which both pressure line23 and also first solenoid valve 25 are connected.

[0020] Operating piston 18 of actuator 16, which is embodied in theexemplary embodiment of FIG. 1 as stepped piston 32 and as a multi-partpiston in FIG. 2, has an effective closing area which, for the closingof gas-exchange valve 10, i.e., the movement of operating piston 18 inthe valve-closure direction, is acted upon by the fluid pressure inpressure chambers 21, 22, and an effective opening area which, for theopening of gas-exchange valve 10, i.e., for the movement of operatingpiston 18 in the opening direction of gas-exchange valve 10, is actedupon by the fluid pressure in pressure chambers 21, 22. Both effectiveareas are made up of various annular surfaces formed on operating piston18 and acted upon by the fluid pressure in pressure chambers 21, 22, aswill also be described below. To achieve defined kinematics of actuator16 during the opening and closing of gas-exchange valve 10, which mustmeet specific demands on gas-exchange valve 10, operating piston 18 isdesigned in such a way that the surface area of the effective areaschange along the sliding path of operating piston 18, namely the surfacearea of the effective opening area upon movement of operating piston 18to generate an opening lift at gas-exchange valve 10, and the effectiveclosing area upon the opposite movement of operating piston 18 togenerate a closing movement of gas-exchange valve 10. These demands madeon actuator 16 are, on the one hand, a high opening force at thebeginning of the opening lift, so that a pressure compensation betweenfront and rear of gas-exchange valve 10 may take place, and asubstantial reduction in the adjustment force following this fraction ofthe overall lift, on the other hand, so that the energy demand requiredto adjust the gas-exchange valve is reduced. Furthermore, a rapidclosing of gas-exchange valve 10 is required as well, wherein thestriking speed of valve-sealing surface 14 on valve-seat surface 15should be as low as possible for noise and wear reasons.

[0021] These demands are taken into account in that operating piston 18is designed in such a way that, when operating piston 18 is moved out ofits valve-closure position, as it is shown in FIG. 1, the effectiveopening area in the leading area of the sliding path is greater than itis in the remaining sliding path and, when operating piston 18 is movedout of its valve-opening position, the effective closing area in the endarea of the sliding path is smaller than it is in the rest of thesliding path. This design of operating piston 18 is realized in steppedpiston 32 shown in FIG. 1 in that the following are provided in steppedpiston 32: a center piston section 321 which has the largest diameterd1; an inner piston section which in each case adjoins center pistonsection 321 at the top and bottom, specifically, a lower inner pistonsection 322 extending through first pressure chamber 21, which has areduced diameter d2 in relation to diameter d1 of center piston section321; and an upper inner piston section 323 which extends through secondpressure chamber 22 and has a reduced diameter d3 in relation todiameter d2 of lower inner piston section 321; and in each case an outerpiston section 324 and 325 which has a larger diameter d4 and d5,respectively, in relation to that of adjoining inner piston section 322and 323 and which adjoins at the end of lower inner piston section 322and upper inner piston section 323. Formed between inner and outerpiston sections 321 and 322, 323, in each case is a transition zone 326and 327 in which the diameter continually increases from diameter d2 ord3 of adjoining inner piston section 322 or 323, respectively, towardlarger diameter d4 or d5 of outer piston sections 324, 325. Instead of alinear increase—as shown in FIG. 1—of the diameter in transition zones326, 327, another geometric design of transition zone 326, 327 may bechosen to thus influence the lift-dependent characteristics of theeffective opening and closing area.

[0022] When, during the lift movement, stepped piston 32 in thedescribed embodiment moves out of its closing position, which is shownin FIG. 1, the effective opening area at the opening onset results fromthe difference of the two annular areas having annular width d1-d3 andthe annular area having annular width d1-d4. Therefore, the effectiveopening area is the resulting annular area having annular width d4-d3 atstepped piston 32. If, following an initial lift, stepped piston 32 hasmoved to such a degree that lower outer piston section 324 or theadjoining transition zone 326 is pushed out of first pressure chamber 21and upper outer piston section 325 or upper transition zone 327 plungesinto first pressure chamber 22, the effective opening area is formedfrom the difference of the annular area having annular width d1-d5 andthe annular area having annular width d1-d2. The effective opening area,thus, is the resulting annular area having annular width d2-d5 atstepped piston 32, which remains unchanged until the end of the openinglift. Since annular width d4-d3 is greater than annular width d2-d5, theeffective opening area is substantially reduced after a fraction of theoverall lift of stepped piston 32.

[0023] During the closing procedure of gas-exchange valve 10 whenstepped piston 32 moves back into its valve-closure position shown inFIG. 1, the effective closing area at the beginning of the closing liftis formed by the annular area at stepped piston 32 having annular widthd1-d2. Prior to the end of the closing lift, lower transition zone 326and adjoining outer piston section 324 plunge into first pressurechamber 21, thereby reducing the effective closing area to the annulararea having annular width d1-d4. Thus, the closing movement of steppedpiston 32 initially occurs with great closing force, due to the largereffective closing area, and with reduced closing force in the end regionof the closing lift, due to the reduced effective closing area. In eachinstance, a high-pressure seal 33 or 34, which is held in workingcylinder 17 and presses against stepped piston 32, seals pressurechambers 21, 22 from stepped piston 32. High-pressure seal 34 of secondpressure chamber 22 is integrated in a cover 35 which seals workingcylinder 17 toward the top.

[0024] A secondary effect is additionally achieved during the closingprocedure in that the diameter of operating piston 32 in second pressurechamber 22 changes shortly before the end of closing, due to theemergence of piston sections 325 and 327, so that the operating-pistonarea delimiting second pressure chamber 22 is enlarged. This causes anincrease in the displacement volume of operating piston 32 in secondpressure chamber 22, which, due to the throttling of the displacementvolume in open second solenoid valve 26, leads to a rapid increase inthe counteracting force opposing the closing movement of operatingpiston 22. This counteracting force brakes operating piston 22 and, incombination with the reduced closing force of operating piston 22,substantially reduces the striking speed of valve tappet 13 onvalve-seat surface 14 of gas-exchange valve 10.

[0025] Actuator 16, schematically shown in FIG. 2 in longitudinalsection, is modified compared to actuator 16 shown in FIG. 1 anddescribed above, to the extent that operating piston 18 is designed insuch a way that, when operating piston 18 moves out of its valve-closureposition, as it is shown in FIG. 2, the effective opening area isreduced by a predefined value following at least one predefined slidingpath and remains constant until the end of the lift, whereas theeffective closing area remains constant when operating piston 18 movesinto its valve-closure position, that is, over the entire closing lift.Thus, gas-exchange valve 10 is rapidly opened with great displacementforce, which then rapidly drops and remains constant over the rest ofthe lift. Instead of actuator 16 in FIG. 1, it is also possible to useactuator 16 according to FIG. 2 in the device described there forcontrolling a gas-exchange valve 10 in combustion cylinder 11 of aninternal combustion engine. The connections of connecting pieces 211,221 and 222 of working cylinder 17 are integrated in the control device,as shown in FIG. 1. Components of actuator 16 in FIG. 2 which correspondto components of actuator 16 in FIG. 1, bear the same referencenumerals, so that in this respect the explanations relating to FIG. 1correspondingly apply to actuator 16 according to FIG. 2 as well.

[0026] The previously mentioned modified design of operating piston 18with the lift-dependent change in the effective opening area is achievedby the fact that operating piston 18 has a plurality of parts and hastwo partial pistons 36 and 37 in the exemplary embodiment of FIG. 2. Thetwo partial pistons 36, 37 have different axial lengths; they areconcentrically inserted inside each other so as to be movable relativeto each another, in such a way that both partial pistons 36, 37 delimitsecond pressure chamber 22 and only inner partial piston 36 delimitsfirst pressure chamber 21. Working cylinder 17 has a stepped design.Upper cylinder section 172, which has a larger diameter, accommodatesboth partial pistons 36, 37, and lower cylinder section 171 of workingcylinder 17 guides only inner partial piston 36. Shorter outer partialpiston 37 is guided in upper section 172 of working cylinder 17 byworking cylinder 17, on the one hand, and by a guide section 361, whichis formed on inner partial piston 36 and has a slightly enlargeddiameter, on the other hand, while longer inner partial piston 36 isguided in lower cylinder section 171 of the working cylinder. Formed bythe cylinder wall of working cylinder 17 is a stop 38 which delimits thesliding path of outer partial piston 37 to sliding path s1, while thesliding path of longer inner partial piston 36 corresponds to theoverall lift s1+s2 of operating piston 18. Inner partial piston 36 iseither integrally formed with a piston rod 39, as this is shown in FIG.2, or pressed onto piston rod 39 as an annular member. Piston rod 39emerges from working cylinder 17 via sealed openings 40, 41. A valvetappet 13 is fastened to piston rod 39. Alternatively, piston rod 39 maybe formed by valve tappet 13 itself.

[0027] When operating piston 18 moves out of its valve-closure position,shown in FIG. 2, in the direction of valve opening, which isaccomplished by applying fluid pressure in second pressure chamber 22,both partial pistons 36, 37 are acted upon by pressure in secondpressure chamber 22 and are moved. The effective opening area ofoperating piston 18 is made up of the two annular end faces of the twopartial pistons 36, 37 and is maximal, the end faces delimiting secondpressure chamber 22. When operating piston 18 has completed valve travels1, outer partial piston 37 strikes against stop 38 and does no longerparticipate in the further displacement movement of operating piston 18.The effective opening area of operating piston 18 is thus reduced to thefront face of inner partial piston 36, which is acted upon by the fluidpressure, so that the displacement force of actuator 16 is reduced andthe energy demand of actuator 16 drops during the further opening ofgas-exchange valve 10.

[0028] If, when reaching the opening position of gas-exchange valve 10,the closing procedure is initiated by discharging first pressure chamber22, a driver pin 42 between the two partial pistons 36, 37 becomeseffective upon inner partial piston 36 having traveled sliding path s2.Inner partial piston 36 takes along outer partial piston 37 via slidingpath S1, up to the closing position of operating piston 18. Driver pin42 is realized by an annular bar 43 which radially projects from theinner side of outer partial piston 37. Guide section 361 of innerpartial piston 36, which has a larger cross section, strikes againstthis annular bar 43. In order to ensure that the fluid passing throughbetween partial pistons 36, 37 drains from upper section 172 of workingcylinder 17, a leakage bore 44 is provided in the housing wall ofworking cylinder 17 in the transition between the two sections 172, 171of working cylinder 17. This leakage bore 44 ends in upper section 172of working cylinder 17 and is used to return the fluid leakage to fluidreservoir 28 via a return line 45. In a further development of thedescribed operating piston 18, it may also be constructed from more thanjust two partial pistons. In that case, the individual partial pistonswill also have different lengths and lose their effectiveness in thefurther movement of operating piston 18 by an appropriate definition oftheir valve travel, so that the effective opening area of operatingpiston 18 changes several times in the course of its overall valvetravel.

What is claimed is:
 1. An hydraulically controlled actuator foractivating a valve, in particular a gas-exchange valve in a combustioncylinder of an internal combustion engine, having two fluid-filledpressure chambers (21, 22) whose chamber volume is controllable, andhaving an operating piston (18) acting upon the valve (10) and beingable to be moved out of a valve-closure position into a valve-openingposition and vice versa, the operating piston (18) delimiting thepressure chambers (21, 22) by piston sides facing away from one another,and having an effective closing area, acted upon by the fluid pressurein the pressure chambers (21, 22) to close the valve (10), and aneffective opening area acted upon by the fluid pressure in the pressurechambers (21, 22) to open the valve (10), wherein the operating piston(18) is designed in such a way that the areal surface of at least one ofthe two effective areas changes along the sliding path of the operatingpiston (18).
 2. The actuator as recited in claim 1, wherein the firstpressure chamber (21), which acts upon the operating piston (18) with afluid pressure in a sliding direction causing a valve closing, ispermanently filled with a pressurized fluid, and the second pressurechamber (22), which acts upon the operating piston (18) with a fluidpressure in a sliding direction causing a valve opening, is able to bealternately filled with pressurized fluid and to be discharged again. 3.The actuator as recited in claim 1 or 2, wherein the design of theoperating piston (18) is such that, when the operating piston (18) movesout of its valve-closure position, the effective opening area is reducedby a predefined value following at least one predefined sliding path. 4.The actuator as recited in claim 3, wherein the operating piston (18)has multiple parts and is made up of concentric partial pistons (36,37), which have differing axial lengths and are able to be movedrelative to each other, and which are inserted into each other in such away that the second pressure chamber (22) is delimited by all, and thefirst pressure chamber (21) only by a portion of the partial pistons(36, 37), and the sliding paths of the partial pistons (37) notdelimiting the first pressure chamber (21) are reduced in a step-wisemanner relative to the overall sliding path of the operating piston(18).
 5. The actuator as recited in claim 4, wherein in each case a stop(38) is positioned in the sliding path of the partial pistons (37) whichblocks the sliding path, the associated partial piston (37) striking thestop (38) after traveling its reduced sliding path (s1).
 6. The actuatoras recited in claim 4 or 5, wherein driver pins (42) are located betweenthe partial pistons (36, 37), which are effective when the operatingpiston (18) is moved out of its valve-opening position into itsvalve-closure position.
 7. The actuator as recited in one of claims 4through 6, wherein in an operating piston (18) assembled from twopartial pistons (36, 37), the outer partial piston (37) has the smalleraxial length, and the inner partial piston (36) is guided in a section(171), having a smaller diameter, of a working piston (17), and theouter partial piston (37) is guided on the inner partial piston (36) andin a section (172) of a working cylinder (17) having a larger diameter.8. The actuator as recited in claim 7, wherein, in the transition of thesection (172) having a larger diameter to the section (171) of theworking cylinder (17) having a smaller diameter (171), a leakage borehas been introduced in the working cylinder (17) which ends in a section(172) having a larger diameter.
 9. The actuator as recited in claim 1 or2, wherein the design of the operating piston (18) is such that, whenthe operating piston (18) is moved out of its valve-closure position,the effective opening area is greater in the leading area of the slidingpath than it is in the subsequent sliding path, and, when the operatingpiston (18) is moved out of its valve-opening position, the effectiveclosing area in the end area of the sliding path is smaller than it isin the preceding sliding path.
 10. The actuator as recited in claim 9,wherein the operating piston (18) is embodied as a stepped piston (32)having a plurality of piston sections (321-325) with different diameters(d1-d5).
 11. The actuator as recited in claim 10, wherein the operatingpiston (18) has a center piston section (321) having the largestdiameter (d1); a lower inner piston section (322) having a comparativelysmaller diameter (d2), lower inner piston section (322) continuing fromthe center piston section (321) and extending through the first pressurechamber (21); an upper inner piston section (323) having a reduceddiameter (d3) in comparison to the diameter (d2) of lower inner pistonsection (322), the upper inner piston section (323) continuing from thecenter piston section (321) and extending through the second pressurechamber (22); and, situated in each case at an end of the inner pistonsections (322, 323), an outer piston section (324 or 325) whose diameter(d4 and d5) is larger than the diameter (d2 and d3) of the adjoininginner piston section (322 and 323).
 12. The actuator as recited in claim11, wherein the outer piston sections (324, 325) are located on theoperating piston (18) in such a way that, upon the operating piston (18)beginning to move out of its valve-closure position, the lower outerpiston section (324) increasingly emerges from the first pressurechamber (21) and, following a stipulated sliding path, the upper outerpiston section (327) increasingly plunges into the second pressurechamber (22) and, toward the end of the movement of the operating piston(18) out of its valve-opening position, the lower outer piston section(324) increasingly plunges into the first pressure chamber (21).
 13. Theactuator as recited in claim 11 or 12, wherein, between each outer andinner piston section (324, 322 and 323, 325), a transition zone (326 and327) is provided at the operating piston (18) whose diameter increasessteadily in a linear manner or following some other mathematicalinterrelationship, from the diameter (d2 or d3) of the inner pistonsection (322 or 323) to the diameter (d4 or d5) of the outer pistonsection (324, 325).
 14. The actuator as recited in one of claims 10through 13, wherein the operating piston (18), by its center pistonsection (321), is guided in an axially movable manner in a workingcylinder (17) forming the pressure chambers (21, 22), and, in the regionwhere the operating piston (18) emerges from the two pressure chambers(21, 22), the operating piston (18) in each case is conducted through ahigh-pressure seal (33, 34), which is affixed in the working cylinder(17) and presses against the operating piston (18).